Method and welding apparatus for the determination of the strength of the welding current to be used in the welding of container bodies

ABSTRACT

For the determination of the welding current to be used for the resistance welding of the overlap seam of container bodies, welding with a test body is carried out with a changing strength of welding current which in the test body produces a varying welding of the seam. The current strength varies from welding of this seam with a too high temperature to welding with a too low temperature. Along with this the welding current strength used in the welding is determined so that it is further determined at which point of the seam the welding has been accomplished and with what strength of welding current. By means of a mechanical and/or optical investigation of the welded seam it can then be easily determined where the seam has been correctly welded for the series production of container bodies from the same sheet material as the test bodies. When such a point or such a region of the seam is known the welding current used in the test welding can be taken as the welding current for serial production.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of the U.S. patent application Ser. No.12/424,042, filed Apr. 15, 2009, which claims the priority of Swisspatent application No. 0605/08, filed on Apr. 17, 2008 the disclosure ofwhich is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The invention concerns a method for determining the strength of thewelding current to be used for the resistance seam welding of containerbodies sequentially following one another. The invention also concerns awelding apparatus for the resistance seam welding of container bodies,with welding rolls, a welding current generator and a welding currentcontrol. The invention further concerns a method for the resistance seamwelding of container bodies with nonuniform overlapping as well as awelding apparatus for the resistance seam welding of container bodieswith nonuniform overlapping.

STATE OF THE ART

For setting the strength of the welding current for the resistance seamwelding of container bodies it is customary to carry out the testwelding of a small series of bodies made of the sheet material to bewelded in the case at hand, wherein as in production the customarilyused welding alternating current is used at a constant current strength.The welded bodies of such small series are then investigated bymechanical strength checking (for example by tearing open the weldingoverlapped seam) and by visual checking. If the welding current strengthin the case of the test welding was set too low, or if the weldingtemperature was too low, the seam can too easily become broken open,since an insufficient material melting was achieved, which is alsodesignated as “adhesion”. On the other hand if the welding currentstrength in the case of the test welding was set to too high, thereresults a welding with a too high temperature, and craters exist in thewelding region and liquid container body or welding electrode materialbecomes deposited as spatter along the seam. This can make the bodyunfit for further use. After such test weldings, in the case of whichthe operating person as a rule creates test bodies with “adhesion”welded seams as well as test bodies with welded seams with spatter, thewelding current strength is then determined for use in the serialproduction of bodies from the involved type of sheet material. In thiscase the welding current strength is set to be in the region between“adhesion” and “spattering” with a spacing of about 2/3 from the lowvalue, and accordingly with a spacing of about 1/3 from the high value.With this iterative procedure the amount of time and the amount ofmaterial consumed by the test series are strongly dependent on the sizeof the welded region of the body material and on the experience of theperson operating the body welding machine.

SUMMARY OF THE INVENTION

The invention has as its object the provision of a better, and therebyas a rule faster and in any event material-sparing, method for thedetermination of the appropriate welding current strength. Further theinvention has as its basis the object of providing a welding apparatusfor carrying out said method.

The object in the case of the above mentioned method is solved in thatat least one test body is welded while during the welding a decreasingor an increasing current strength is used, and in that in said weldingit is determined where on the seam which welding current strength hasbeen active. The object in the case of the above mentioned weldingapparatus is further solved in that by the control and welding currentgenerator the welding current strength during the welding of the seam ofa container body is changeable and its value along the length of theseam of the body is determinable and thereby storable in a memory.

The test body accordingly exhibits a seam which along its seam lengthhas been welded with different high welding current strengths. An entirecurrent strength region, which preferably comprises the region from“adhesion” to the creation of “spatter”, can be “mapped” from this onebody or at most from two bodies. The mechanical and optical, especiallythe visual, investigation of this body shows thereby, as a rule, thatalong its seam is a seam region which has been welded with anappropriate strength of welding current. If this is not the case, then asecond test body with a different current strength region can beproduced in accordance with the method of the invention. Then, since thewelding current strength is known as to the value which has been appliedalong the length of the seam, from the position of the correctly weldedregion of the test body the corresponding welding current strength canbe determined, and within that correctly welded region a spot on theseam can be chosen (for example a spot which divides the correctlywelded region between “adhesion” and “spattering” in a ratio of 2/3 to1/3), and the welding current strength which was effective at this spotcan be determined, especially as read off from of the ramp of thewelding current characteristic or as read out from the memory. Thiswelding current strength can then be taken as the determined weldingcurrent strength to be used for the serial production welding at aconstant current strength value.

In a preferred embodiment the current strength variation along the seamof the test body is not steady but instead has at least one inflexionpoint with different slopes on both sides of the inflexion point. Inregard to the test body this allows that in the region of the higherwelding current strength a lower slope of the current strength change isused and that in the region of the smaller welding current strength asteeper slope of the welding current strength change is used. Thisprovides a better recognition of the “spatter” boundary and of a goodtearing open capability of the seam in the “adhesion” region. Preferablythe welding starts with a high current strength, and accordingly thewelding current strength falls in going along the length of the seam. Itis further preferred that the welding current is first started at apredetermined spacing, for example 5 to 10 millimeter, from thebeginning of the body, so that possible vibrations of the welding rolls,or jitter effects, do not falsify the measurement of the welding currentstrength along the seam.

According to a further aspect of the invention, in the case of welding aproduction series of container bodies, by electrical resistance seamwelding, where the container bodies have a nonuniform overlapping it isseen to that in the production welding of each such body along thelength of its seam a falling or rising welding current strength is used.This permits a welding in the regions with higher overlapping withhigher current strength in comparison to a welding in regions withsmaller overlapping of the body edges, where then the welding takesplace with lower current strength. The determination of the optimalwelding current strength curve takes place preferably with the methodaccording to the above first aspect of the invention. Accordingly withthis method (therefore with falling or rising welding current strength)for two or more bodies with differing overlapping the optimal currentstrength values for the welding in the region of larger overlapping andfor welding in the region of lesser overlapping are determined. Thewelding current strength for production is then chosen as a rule to be aramp shaped current strength curve which connects these two currentvalues. The second aspect of the invention also includes a suitablewelding apparatus for carrying out this method.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, exemplary embodiments of the different aspects of theinvention are explained in more detail with the aid of the drawings. Thedrawings show:

FIG. 1—a schematic view of a welded seam of a container body and thecurve of the welding current strength (for example in amperes) withrespect to the length of the seam or with respect to time;

FIG. 2—a further schematic illustration of a welded seam of a containerbody and the curve of the welding current strength, for explaining apreferred embodiment of the invention;

FIG. 3—a schematic illustration of a welding apparatus for theresistance seam welding of container bodies according to the aspects ofthe invention; and

FIG. 4—a further schematic illustration of a welded seam of a containerbody and the curve of the welding current strength, for explaining thesecond aspect of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows schematically a test container body 2, which serves for thedetermination of the appropriate welding current strength value to beused for the serial production of bodies made from the same sheetmaterial as the test body, above a diagram which illustrates the weldingcurrent strength along the length of the seam of the test body 2, orcorrespondingly in respect to the time needed for the welding. The testbody 2 is welded on a resistance seam welding machine or roller seamwelding machine, which is explained in more detail in connection withFIG. 3. According to the invention now the strength of the weldingcurrent during the test welding of the body 2 along the overlappedwelding seam is varied so as to fall or rise. In the illustrated examplethe welding current strength 30 falls during the time of the welding, oralong the length of the seam, and accordingly diminishes from a startingvalue to an ending value. The welding current in this case, as in knownways for container body welding, is an alternating current, and as arule has a higher frequency then the frequency of the power supplyvoltage, as is known to persons skilled in the art and is not hereexplained further. It is new however, that in the welding of the testbody the strength of the welding current applied to the overlapped seamof the test body is varied. Accordingly, along the length of the weldedseam of the test body 2 from its beginning 3 to its end 4 (withreference to the transport direction of the body) an overlapped weldingseam is created with a region 6 formed with a too high weldingtemperature and containing the appearance of “spattering”, as well aswith a region 5 formed with a too low welding temperature and at which acold welding seam or so called “adhesion” seam is created having aninsufficient amount of welding. Between these regions 6 and 5, therethen lies, because of the variation of the welding current strength, awelding region 19 in which a welding without spattering and with asufficient welding temperature is created. In the illustration thehigher welding current strength is used at the beginning of the body andthe welding current 30 or its strength falls to the end of the seam orto the end 4 of the body. Of course a reverse process is also a possiblein which a lower welding current strength is used at the beginning 3 ofa body and in which the welding current strength increases along thelength of the seam. Then the region 5 would be found at the beginning 3of the body and the spatter region 6 would be found at the end 4 of thebody. Between those points again a correct welded region 19 would beformed. Accordingly, by way of the invention a test body 2 can be formedwhich in the seam of a test body 2 there is represented an entirewelding current region extending from too high a current weldingstrength to too low a welding current strength. Should the operatingperson have falsely chosen the beginning setting of the welding currentstrength so that this imaging is not produced, as the case may be, it ispossible to carry out a further test body with a lower or higher valueof the beginning welding current strength so that in the illustrateddiagram of the welding current strength the ramp is moved paralleldownwardly or upwardly. In any event the steepness of the weldingcurrent strength 30 along the length of the seam, or in respect to thewelding time, can be adjusted, as in indeed is preferred if a pregivensteepness of the ramp is input into the control. If a test body 2 isproduced in accordance with FIG. 1 and having the seam areas 6,5 and 19,then on the basis of an optical or visual examination of the presence ofspattering the boundary between the region 6 and the region 19 can bedetermined. Further in a known way by the tearing open of the seam fromthe end 4 the region 5 can be determined and this above all can besupplemented by an optical or visual inspection. In this way theboundary between the region 5 and the region 19 is determined. Thewelding current strength to be used for the production of containersfrom the same sheet material as used for the test body can then be foundto lie in the welded region 19. This is possible since in the formationof the test body the welding current strength 30 along the length of theseam has been determined. This can be done by an actual measurement ofwelding current strength along the seam during the welding of the testbody and the storing of the measured values, so that the actual weldingcurrent strength corresponds to the determined welded current strength.This can also be done in that in the control is arranged a pregivenramp-shaped desired curve for the welded current strength along theseam, or with respect to time, so that not a measured value but apregiven value for the welding current generator is given. Therefore, iffor the test body 2 the good welding region 19 has been determined, andif a welding is desired as is present for example at the point A of theregion 19, which is found, for example, by dividing into three thelength of the region 19, and fixing the point A at a distance of onethird from the boundary between the areas 19 and 6, so how large thecurrent strength at the welding at point A should also remain fixed withthe distance of the point A from the beginning of the body or from thebeginning of the application of the welding current. This weldingcurrent strength at the point A, determined as said by measurement or byway of the predetermined current strength along the welding seam, whichcan for example be called up, by the input of the seam length from thebeginning of the body 3 to the point A, from the measured-value memoryin the control 20 (FIG. 3), or from the pregiven value of the rampcontained in the memory, since according to the invention the currentstrength values along the length of the seam have been determined.Accordingly, by a simple length measurement, using a measuring rule, thebody length from the beginning 3 to the point A can be determined, andfrom it the current strength suitable for production usage can bedetermined. This can then be set into the welding machine for serialproduction or can be directly taken from the memory, in which memory thefindings resulting from the test welding have been stored or in whichthe preset desired values along the seam have been stored. Since theseam length and time stand in fixed relation to one another for thepregiven welding speed of the welding machine, in the acquisition eitherthe welding time or the seam length can be determined, either by way ofthe rotational angle of the welding rolls during the welding of the testbody 2 or by way of the used length of the intermediate wire electrode11, 12 (FIG. 3).

In a preferred embodiment the procedure is such that the high current isused at the beginning, as is illustrated in FIG. 1. Further, it ispreferred that the current is first applied after the start of theappearance of the body at the welding rolls, and indeed most preferablyabout 5-10 mm beyond the beginning 3 of the body 3, so that possibletransient starting movements or fault containing synchronizationadjustments in the measurement of the welding current cannot have anyinfluence. The welding machine can for this be equipped with a detectorfor detecting the body beginning 3, as will be explained later.Preferably the welding current is further turned off shortly before theend 4 of the body 2. This simplifies the tearing open of the seam forthe mechanical inspection.

FIG. 2 shows a further preferred embodiment for which the precedingexplanations also serve. Similar reference numbers indicate againsimilar elements. In the case of this embodiment the welding takes placenot with a constantly falling or constantly rising current 30, but witha current curve which has at least two different slopes and acorresponding inflection point (as illustrated) or has severalinflection points. In FIG. 2 two sections 31 and 32 of the currentstrength which occur along the seam or with respect to time areillustrated. In the region of the higher current the slope of thedecreasing ramp is less and in the region of lower current 32 the slopeis larger. In this way it can be achieved that the spatter boundary onthe seam between the regions 6 and 19 is visible with a higherresolution and the seam at the end 4 can nevertheless be well torn opento ascertain the region 5.

FIG. 3 shows schematically a resistance seam welding machine or rollseam welding machine 1 which includes two welding rolls 7 and 8, betweenwhich the seam of body 2 is welded in a known way. Intermediate wireelectrodes 11 and 12 can run on the welding rolls. The lower weldingroll 8 is supported on a welding arm 10 and is supplied by the conductor15 from the welding current generator 14 through the welding transformer17. By a sensor 21, especially a noncontactingly operating sensor, or bya detection system integrated in the welding current generator it can bedetermined when the body 2 is located between the welding rolls. Thewelding current generator then controls the application of the weldingcurrent through the conductor 1 to the upper welding roll 7. Thereby,especially, the delayed application of the welding current at a smallspacing from the body beginning can be controlled. The controller 20 canfurther control the welding current generator 14 and can impress on thisthe desired ramp shaped curve of the welding current strength, which inthis case is contained in the memory of the control 20 as pregivenvalues. The ramp shaped curve can however also be set into the weldingcurrent generator itself and can be activated by an external signal forthe test body welding. A processor controlled welding current generator,which makes possible a process according to the invention is a weldingcurrent generator Pulsar or Unisoud from the firm Soudronic,Bergdietikon, Switerzland. The control contains further the welding timefor the test body or the welded seam length, as has been previouslyexplained. The control 20 can also further determine the currentstrength supplied to the current welding rolls with respect to time orwith respect to the seam length, whether this is done by measurement orby comparison of the welding time or welding seam length with thecorresponding desired current strength of the ramp. In FIG. 3 it isillustrated as a variation that the conductors between the control 20and the generator 14 can also, in opposite direction, deliver a currentstrength output signal from the generator to the control 20.Alternatively a separate current strength measuring device 26 can beprovided which delivers the strength of the welding current to thecontrol whereby this is captured and associated with the welding time orthe seam length. Alternatively, the measurement is processed insteadwith the desired value ramp in the control or in the generator for thetest body welding current strength. By the determination of a preferredspot A in the correct welding region 19 of the welding seam of the testbody 2 this can then, as explained, be associated with the correspondingwelding current strength according to the ramps 30 or 31, 32, which aremeasured ramps or pregiven desired current strength value ramps. Thisone current strength value is then from the control 20 used as thewelding current desired value for the production series welding ofcontainer bodies. Accordingly in known ways with a constant, not fallingand not rising, welding current the welding takes place with thedetermined welding current strength. Thereby the production welded seamslie in the optimal region corresponding to the point A on the weldedseam of the test body.

With reference to FIGS. 3 and 4 a further aspect of the invention can bedescribed, which aspect is explained as a method with reference to FIG.4, and which method can be carried out with the welding apparatus ofFIG. 3. It can happen that in the production of the body blanks, by thecutting of the sheet material and by the rounding of the body blanks,the overlapping edge region of the container body does not run uniformlyalong the welding seam to be formed. In FIG. 4 a corresponding body isschematically illustrated, wherein at the beginning 3 of the body, atleast in the region 35, the body has an overlapping which is too large,and at the body end 4, at least in the region 36, the body has anoverlapping which is too small. With customary procedures such containerbodies can hardly be welded with sufficient quality, since the constantproduction welding current strength in the large overlap as a ruleprovides insufficient energy, so that there a too cold welding isachieved, and on the other hand since the constant welding currentstrength in the too small overlap supplies too much energy, so thatthere is seen the effect of welding with a too high temperature and withconsequent spattering. According to the second aspect of the inventionthe procedure is now such that the welding in production takes placewith a corresponding decreasing welding current strength, as isillustrated in FIG. 4 beneath the body, where again the welding currentstrength is shown on the vertical axis and the welding time or seamlength is shown on the horizontal axis. If the overlap fault of thecontainer body 2 runs otherwise so that at the beginning 3 a too smalloverlap is present and at the end 4 a too large overlap is present, thenthe welding current strength must accordingly run in an increasingmanner instead of as shown in FIG. 4, in a falling manner. The weldingmachine of FIG. 3 is equipped with the control 20 and the weldingcurrent generator 14 (which again for example is of the type Pulsar orUnisoud of the firm Soudronic AG, Bergdietikon, Switzerland)corresponding to the second aspect of the invention, so that in seriesproduction for each of the bodies which rapidly follow one another acorresponding falling or increasing welding current strength is createdalong the involved seam.

For the determination of the desired welding current strength at thebeginning and at the end of the seam preferably the methods explained inreference to FIGS. 1 to 3 of the first aspect of the invention are used.Therefore with these explained methods, which are used for two or morebodies, for the body start and for the body end with dissimilaroverlapping widths, for each end another optimal current is determined.These two current values for the beginning and for the end of thewelding seam are then in the serial production of bodies introduced intothe welding current ramp profile illustrated in FIG. 4, so that theoverlap deviation can be compensated. Again the optimal welding currentfor the body beginning and the body end can in customary ways bedetermined by the welding of a series of test bodies with differentconstant welding currents.

For the determination of the welding current strength for the resistanceseam welding of the overlap seam of container bodies in accordance witha first aspect of the invention with a first test body a welding with achanging welding current strength is carried out, which in the case ofthe test body produces a varied welding of the seam which weldingextends from a welding with a too high temperatures to a welding with atoo low temperature along the seam. Thereby the welding current strengthfor the formation of the welding is determined (by measurement or fromthe presettings) so that it is determined at what spot on the seam theseam has been welded with what welding current. By means of a mechanicaland/or optical examination of the welding seam it can then easily bedetermined where the seam has a correct welding for the serialproduction of the container bodies made of the same sheet material asthe test bodies. When one such location or one such region of the seamis known then the welding current strength determined with the testwelding can be used for the serial production wherein the welding takesplace with a constant welding current strength.

Even a person operating the resistance seam welding machine and havinglittle experience in welding technology can, with the aid of theinvention and without special knowledge about the system characteristics(for example production rate, welding pressure) and about the nature ofthe sheet material to be processed, in little time make a qualitativestatement about where the welding region for the production series lies(minimum, maximum, and optimum welding current strength). Since for thedetermination only one to two test bodies are needed, the method of theinvention requires considerably less time and less material then thecustomary process.

According to a further aspect of the invention in serial production thewelding results in the making of a series of container bodies with afalling or a rising welding current strength curve for each body, sothat the bodies with a nonuniform running of the overlap of the bodyedges can be handled. This permits the welding of such bodies with goodwelding quality, which in the customary way with constant weldingcurrent strength is not possible.

The invention claimed is:
 1. A welding apparatus for resistance weldingof a container test body, comprising: electrode welding rolls betweenwhich container test body passes during a resistance welding operationin which a weld seam is generated along the test body as the test bodypasses between the welding rolls, a welding current generator connectedwith and supplying a welding current to the welding rolls and a weldingcurrent control connected in controlling relationship to the weldingcurrent generator, a memory in the welding current control configured tocontain given welding data including a given welding current curvedefining a welding current strength, variable as a function of length ofthe weld seam produced during the resistance welding operation on thecontainer test body, during which operation the container test bodymoves between the electrode welding rolls at a welding speed, wherebythe welding current control is caused to vary the strength of a weldingcurrent applied by the generator to the electrode welding rolls alongthe weld seam on the container test body during the resistance weldingof the container test body according to the given welding current curve,and thus the weld current strength varies from position to positionalong the weld seam and the welding current strength employed at eachposition along the weld seam is determined from the welding currentcurve and the position along the seam length, or the welding time andthe welding speed.
 2. The welding apparatus according to claim 1,wherein the welding current strength is determinable at each positionalong the length of the seam by way of a detector of a rotation angle ofat least one of the welding rolls or a used length of an intermediatewire electrode.
 3. The welding apparatus according to claim 1, furthercharacterized in that the given welding current curve along the seam hasat least two current strength regions with differing steepness of afalling or rising of the current strength.
 4. The welding apparatusaccording to claim 1, further characterized in that the welding currentis applied to the container test body after a detector detects theappearance of the body between the welding rolls.
 5. A welding apparatusfor the electric resistance seam welding of a container test body,comprising: welding rolls, a welding current generator applying weldingcurrent to the welding rolls, a welding current control controlling thegenerator, and a memory in the control containing given welding dataincluding a given welding current curve defining a welding currentstrength which is employed by the welding current control to control thewelding current generator, and which varies from position to positionaccording to the welding current curve along a weld seam on thecontainer test body for a given welding time during which the containertest body moves at a given welding speed, whereby the welding currentcontrol causes the strength of the welding current applied to thewelding rolls during the welding of the container test body to changealong each portion of the weld seam on the container test body accordingto the given welding current curve, and the weld current strengthemployed at each position along the weld seam can be determined from thewelding current curve and the seam length, or welding time and the givenwelding speed, and is available for determination of a suitable weldingcurrent strength for welding a production container body.
 6. The weldingapparatus according to claim 5, characterized in that by way of thewelding current control and the welding current generator the weldingcurrent strength during the welding of the container test body or ofseveral container test bodies in accordance with given welding curve ismade to fall or rise.
 7. The welding apparatus according to claim 5,wherein the welding current strength is determinable along the length ofthe seam by way of a detector of a rotation angle of at least one of thewelding rolls or an intermediate wire electrode used for the welding. 8.The welding apparatus according to claim 6, further characterized inthat the welding current curve along the weld seam has two currentstrength regions with different steepness of the falling or rising ofthe current strength.
 9. The welding apparatus according to claim 5,further characterized in that the welding current is applied after apredetermined delay following the appearance of the container test bodybetween the welding rolls.
 10. A welding apparatus as defined in claim4, wherein the detector is a sensor at the welding rolls to detect theappearance of the container test body between the welding rolls.
 11. Awelding apparatus as defined in claim 4, wherein the detector is part ofthe welding current generator.